1. Field of the Invention
The present invention relates to stored charge control in a junction transistor, and more particularly to the use of a "recombination layer" diode to feedback clamp a junction transistor and thereby significantly reduce the transistor turn-off time.
2. Description of the Prior Art
One of the most common applications of a junction transistor is an on-off switch, somewhat analogous to a single pole, single throw electromechanical switch. In such application, a control voltage applied to the transistor base is used to switch the transistor from an OFF condition in which there is substantially no collector-emitter current, to an ON condition in which heavy current flows in the collector-emitter circuit.
For high speed switching operations it often is desirable to overdrive a junction transistor, thereby materially reducing the turn-on or rise time of the collector current. However, a penalty is accrued when such a transistor is overdriven to the on condition. When a junction transistor is saturated, the collector junction is forward biased. Thus, the collector is emitting carriers into the base, and as a result the density of excess minority carriers in the base next to the collector junction is abnormally large. When an attempt is made to turn off such a saturated transistor, the collector current cannot decrease until the excess minority carrier density near the base collector junction is reduced nearly to zero. Moreover, removal of these excess minority carriers in itself tends to support the current level in the collector leg during this turn-off time. Thus, a dead time or storage interval must transpire before the transistor starts to turn off.
A number of approaches have been used in the past to decrease the storage delay time in a junction transistor. For example, the transistor junctions may be gold doped to enhance carrier recombination. By providing additional recombination centers, the gold dopant ions speed up the clearing of excess minority carriers from the base region when the signal voltage supplied to the transistor base is turned off. While improving the transistor switching speed, gold doping significantly deteriorates other transistor characteristics such as minimum forward (collector-to-emitter) voltage drop, and minimum collector-base leakage.
Another approach of the prior art is to employ a clamping circuit to clamp the collector-emitter voltage drop of the transistor to a value somewhat greater than zero during the ON period to avoid saturation. Conventionally, such clamping circuits have utilized a diode forward connected between a clamping voltage source and the collector of the transistor, or alternatively, have employed a breakdown diode connected across the collector load.
While some feedback clamping circuits have been suggested, these normally require a diode fabricated of a semiconductor material having a significantly different band gap than that of the transistor semiconductor material. For example, when using a silicon transistor, a germanium forward clamping diode is required. Alternatively, a series-connected base resistor must be empolyed between the clamping diode and the transistor base. This clamping configuration not only requires an additional circuit component, but also necessitates higher drive voltage and/or current for circuit operation.
Thus no simple approach is known in the prior art for effectuating a significant reduction in the turn-off time of a junction transistor without the drawbacks enumerated above. In contradistinction, the present invention provides means for decreasing the on-to-off switching time of a junction transistor, while overcoming these and other shortcomings of the prior art.